That's the Mosquito Anti Loitering Device
http://www.movingsoundtech.com/
It a high frequency that only young ears can hear that sounds like the whine of a mosquito. Old ears lose frequency range so aren't bothered. It's not harmful, just annoying.
http://www.movingsoundtech.com/
It a high frequency that only young ears can hear that sounds like the whine of a mosquito. Old ears lose frequency range so aren't bothered. It's not harmful, just annoying.
In London, TFL has found the same effect can be hard without some discrimination by playing classical music.
That's my retirement plans anyway.
For example, if you assume that someone will be under it for 8 hours (an employee during a work day), the OSHA approved exposure is 90dB, which will likely not be annoying enough to serve its purpose.